1. Field of the Invention
The present invention concerns a method and system for generating an x-ray exposure of an examination subject exhibiting a curved geometry.
2. Description of the Prior Art
Methods are known for generating an x-ray exposure of an exposure subject extending in a longitudinal direction which, at least in sections, exhibits a curved geometry in a main projection direction transverse to the longitudinal direction, wherein initially partial exposures of individual sections of the exposure subject are generated by positioning of an x-ray source and an x-ray detector at various positions along the longitudinal direction of the exposure subject, and then the partial exposures of the sections are combined into an overall x-ray exposure of the exposure subject. Moreover, x-ray systems are known for generating an x-ray exposure of such an exposure subject; which have an x-ray source and an x-ray detector that can each be moved at least along the longitudinal direction of the exposure subject; under the control of a control device, to generate partial exposures correlated to individual sections of the exposure subject; and having an image combination device that forms an overall x-ray exposure of the exposure subject from the partial exposures, and/or with means to identify the partial exposures for a later combination.
Digital x-ray detectors have been changing classical radiography for years. Among the newer technologies that have been in use or are just about to come to market are image intensifier camera systems based on television or CCD cameras, storage film systems with integrated or external reader unit, systems with optical coupling of the converter film to CCDs or CMOS chips, selenium-based detectors with electrostatic readout and solid-state detectors with active readout matrices. Common to all of these technologies is that the x-ray information ultimately exists digitally both as to position, in the form of a two-dimensional pixel structure, and in the signal amplitude in the form of grey scale values with given bit depths.
Such digital x-ray systems enable a series of new applications such as, for example, the representation of organs that are very much larger than the size of the detector surface of the detector that is used. In classical radiography, such exposures are made with the aid of oversize x-ray films at a larger distance of the exposure subject from the x-ray radiator. In contrast to this, with digital detector systems, it is possible to acquire an exposure series in the above-specified manner with individual partial exposures of the examination subject, and subsequently to combine the individual images with an image fusion method, so that a large overall exposure results, A greater flexibility is achieved with such digital methods, and a better image quality is achieved than with classical method.
To generate an exposure series, the x-ray source (also called x-ray radiator in the following) and the x-ray detector matched thereto are shifted in a straight line along the longitudinal direction of the exposure subject, and thereby produce partial exposures at specific positions. Normally, care is taken that the individual exposures of adjacent partial sections respectively overlap somewhat at their ends. This simplifies the precise positioning of the individual partial exposures relative to one another for the combination of the individual partial exposures into the desired overall x-ray exposure of the examination subject. Examples of such examinations in which larger exposure subjects (normally extending in a longitudinal direction) are acquired by a number of partial exposures, and from this an overall exposure is formed, are whole-body exposures or exposures of arms, legs, vessels or the spinal column of a patient. It is always important in such x-ray exposures that the exposed subject be correctly imaged as precisely as possible with regard to his individual structures. This is particularly a problem given complicated skeletal structures such as the spinal column. In the case of a spinal column exam, in addition to the shape, in particular the structure at the edge of the vertebral body and the intervening space between the individual vertebrae, meaning the representation of the intervertabral discs, are of significant importance. With the known type of generation of the individual partial exposures by means of an x-ray source moving in a straight line and an x-ray detector running in parallel, shadow formations are disadvantageously created directly in the edge regions and the intervening spaces between the vertebrae due to overlapping of adjoining vertebrae and due to parallax effects. This can lead to defects that are not clearly associated with a specific vertebra. A precise association of the defects with a specific vertebrae or with specific vertebrae is, however, indispensable for therapy. This means the overall x-ray exposure of a spinal column generated with a known digital acquisition method normally exhibits the same shadow formation of the individual vertebrae at the edge regions as is also the case given the classical acquisition method, in which the entire spinal column is recorded on an oversize x-ray film. A similar problem is represented by exposures of other large skeletal structures, in particular in the joint areas.